As is known, in an optical microscope of a confocal scanning type, spot illumination from a spot light source is irradiated on a sample, with the spot illumination operated so as to scan X- and Y-directions, and light transmitted through the sample or light reflected from the sample is converged onto a pin hole. The intensity of light transmitted through the pin hole is detected by a photo detector, thereby to perform a measurement of information concerning the surface of the sample.
In some cases, this kind of confocal scanning type optical microscope is used in combination with an image rotation device, in order that the scanning direction of spot illumination can be oriented in an arbitrary direction relative to a sample with the sample kept fixed. The image rotation device has an image rotation prism for rotating an optical image at an arbitrary angle in a plane vertical to the optical axis.
The image rotation prism is arranged such that a light beam entering thereinto is reflected in the prism for odd-numbered times and is thereafter emitted to the outside. Therefore, an image obtained by an emitted light beam is inverted upside down but is not changed laterally. By rotating the image rotation prism by an angle of .theta. around the optical axis of the entering light as the center, an optical image obtained by emitted light can be rotated by an angle of 2.theta. around the optical axis.
Also, in the confocal scanning type optical microscope, there is a case that a user wants to zoom in an arbitrary portion of a sample, to observe the sample. It has been conventionally considered that a structure which zooms in (or magnifies) and positions an arbitrary portion of a sample with ease and with high accuracy adopts a pair of galvanic mirrors capable of performing X-scanning and Y-scanning. Specifically, an arbitrary portion of a sample can be magnified by zooming-in in a manner in which the center angles of swings of the galvanic mirrors are changed by a predetermined angle and the scanning range in the X- and Y-directions are offset to change the scanning width.
In the confocal scanning type optical microscope, the following problems should be solved.
Firstly, in a conventional structure, the polarization characteristic concerning light entering into an image rotation prism and light exiting from the prism changes in accordance with rotation of an image rotation prism. Therefore, a confocal scanning type optical microscope of a reflection type results in a problem that the brightness of an image observed is greatly changed due to the change of the polarization characteristic.
For example, in case of the structure as described above, only the prism is rotated while the polarization characteristic of the entering light is fixed. This means that the polarization characteristic of the exiting light is relatively rotated. In this case, if the polarization characteristic of exiting light from the prism changes, the amount of light reflected by a deflection splitter accordingly changes, so that the brightness of an image observed greatly changes, in a scanning type optical microscope of a reflection type in which light reflected from a sample is separated from entering light to efficiently introduce light reflected from the sample to a detector, by using a linearly polarized laser is used as a light source and by combining a deflection beam splitter and a .lambda./4 plate with each other.
Secondly, the following problem occurs when an image is magnified with use of galvanic mirrors described above while rotating the image by a prism. Specifically, in the above structure, the scanning range is offset and the scanning width is changed, by controlling the scanning angles of the galvanic mirrors for scanning the X- and Y-directions. Zooming of an arbitrary portion of a sample is thus performed. However, if the prism is rotated by 90.degree. in this condition, there occurs a problem that the scanning range is shifted to a quite different range since the center of rotation and the center of the scanning range are offset.
Thirdly, where image rotation is performed with use of a prism, the axes of entering light and exiting light may be different from each other depending on the processing accuracy of respective surfaces of the prism, so that so-called whirling of light may occur.
More specifically, in case of using an image rotation device, an optical axis of the prism may be inclined to a reference plane (or a reflection surface) of the prism, depending on the processing accuracy of respective surfaces of an image rotation prism forming part of the image rotation device, or a wedge error in a prism surface may cause a difference between the optical axis of entering light and the optical axis of exiting light (i.e., the prism itself has no optical axis).
In addition, any of three axes of the rotation axis of a rotation mechanism, the optical axis of entering light, and the optical axis of the prism may be different from the other or others, due to the processing accuracy or the assembling accuracy of components of a rotation mechanism.
If an error occurs as described above, the light emitted from the image rotation device is derived from a certain optical axis, but has an angular difference .delta. .theta.. Besides, the amount and orientation of the light change in accordance with image rotation by the image rotation device, so that so-called whirling of light may occur.
As a result, an image taken in by an image detector is not rotated around the center of an optical axis, thereby causing a factor which displaces the position of an image in accordance with rotation of the image. When measuring a fine small line width as described above, there is a problem that the positional displacement causes a measurement target to be positioned out of a measure area, so that measurement is impossible. In addition, not only in measurement of a fine small line width, but also in any optical system having an image rotation device, there is a problem that some portion may falls in a peripheral portion of an image observed, due to a displacement of the center of an image.
It may be considered that the processing accuracy of respective surfaces of an image rotation prism, and the processing accuracy and assembling accuracy of a rotation mechanism should be improved much more. However, such improvements will increase the manufacturing costs and are therefore not: preferable for practice.